1. Field of the Invention
The present invention relates to dielectric ceramic compositions for use as resonators employed in a microwave frequency band of several gigahertz.
2. Description of the Prior Art
The recent increase in information density results in a progressively increased signal frequency used. Particularly, a microwave having a frequency of several hundred megahertz to several gigahertz is used in various information transmission media such as satellite communication, microwave remote communication, broadcasting, and further a microwave remote recognition system.
A resonator or a filter for a transmitter-receiver is indispensable in the media, and is manufactured using a dielectric ceramic material adequately performing a function in its high frequency band.
Examples of this type of dielectric ceramic material conventionally used include dielectric ceramic materials of a BaO - TiO.sub.2 system, a Ba {Zn.sub.1/3 (Nb.Ta).sub.170 }O.sub.3 system, a (Zr.Sn) TiO.sub.4 system, and the like for the reason that the frequency characteristics are relatively good.
In the resonator or the like manufactured using the dielectric ceramic material, if the dielectric constant of its dielectric is taken as .epsilon., the wavelength of an electromagnetic wave propagating through the dielectric is small, i.e., ##EQU1## Consequently, the higher the dielectric constant (.epsilon.) of the material used is, the smaller the size of the electronic component such as the resonator can be.
However, the dielectric constant of the above described dielectric ceramic material is generally low, i.e., 20 to 40. Accordingly, the size of the resonator is increased in a microwave frequency band of 1 to 3 GHz.
Examples of a dielectric ceramic material having a higher dielectric constant (.epsilon.) than the above described dielectric ceramic material include SrTiO.sub.3 .epsilon.; about 300) and CaTiO.sub.3 (.epsilon.; about 180). The temperature coefficients of resonance frequency (.tau.f) of the materials are respectively very high, i.e., +1700 ppm/.degree. C. and +800 ppm/.degree. C., so that the stable use cannot be expected.
Therefore, examples of a method of developing a dielectric ceramic material having a high dielectric constant and having a temperature coefficient which is close to zero include a method of mixing a dielectric ceramic material having a high dielectric constant and having a temperature coefficient which is large on the positive side and a dielectric ceramic material having a high dielectric constant and having a temperature coefficient which is large on the negative side. Examples of a material developed in such a method include a dielectric ceramic material of a LiO.sub.2 - CaO - E.sub.2 O.sub.3 - TiO.sub.2 system (where B is Sm or Nd) which is disclosed in U.S. Pat. No. 5,188,993. In putting the dielectric ceramic material to practical use, however, a material having a temperature coefficient which is close to zero, having such a Q value as not to interfere with practical use, and having a higher dielectric constant and a material having three characteristics such as a dielectric constant, a temperature coefficient and a Q value which are flexibly varied depending on the use have been required at the present time.